相信伙伴们在日常的开发工作中，一定会遇到事件冲突的问题，e.g. 一个页面当手指滑动的时候，会翻到下一页；点击的时候，需要响应页面中的元素点击事件，这个时候如果没有处理滑动事件，可能遇到的问题就是在滑动翻页的时候却只响应了点击事件，这个就是点击事件与滑动事件的冲突。其实还有很多常见的经典事件，e.g. RecyclerView嵌套滑动，ViewPager与RecyclerView嵌套滑动等，所以这个时候我们需要对事件分发非常了解，才能针对需求做相应的处理。 # 1 Android 事件分发机制这是一个老生常谈的问题，相信伙伴们都了解常见的Android事件类型：ACTION_DOWN、ACTION_MOVE、ACTION_UP，分别代表手指按下屏幕的事件、手指滑动的事件以及手指抬起的事件，那么从手指按下到事件响应，中间经历了什么呢？我们从Google的源码中去寻找答案。 ## 1.1 事件分发流程因为对于组件来说，这个事件要么消费要么不消费（事件处理），而对于容器来说，还需要做的一件事就是分发事件，通常是先分发后处理，而View就只是处理事件。 ![image.png](https://p1-juejin.byteimg.com/tos-cn-i-k3u1fbpfcp/317c6a43493f430490fea3410a12a08a~tplv-k3u1fbpfcp-watermark.image?) 因此在进行事件冲突处理的时候，对于事件是否向下分发给子View消费，就需要在父容器中做拦截，子View仅做事件消费。 ## 1.2 View的事件消费首先我们先不看事件是如何分发的，先关注下事件是如何被处理的，在View的dispatchTouchEvent方法中，就包含对事件的处理全过程。 ```java public boolean dispatchTouchEvent(MotionEvent event) { //...... boolean result = false; if (mInputEventConsistencyVerifier != null) { mInputEventConsistencyVerifier.onTouchEvent(event, 0); } final int actionMasked = event.getActionMasked(); if (actionMasked == MotionEvent.ACTION_DOWN) { // Defensive cleanup for new gesture stopNestedScroll(); } if (onFilterTouchEventForSecurity(event)) { if ((mViewFlags & ENABLED_MASK) == ENABLED && handleScrollBarDragging(event)) { result = true; } //核心代码1 ListenerInfo li = mListenerInfo; if (li != null && li.mOnTouchListener != null && (mViewFlags & ENABLED_MASK) == ENABLED && li.mOnTouchListener.onTouch(this, event)) { result = true; } //核心代码2 if (!result && onTouchEvent(event)) { result = true; } } if (!result && mInputEventConsistencyVerifier != null) { mInputEventConsistencyVerifier.onUnhandledEvent(event, 0); } // Clean up after nested scrolls if this is the end of a gesture; // also cancel it if we tried an ACTION_DOWN but we didn't want the rest // of the gesture. if (actionMasked == MotionEvent.ACTION_UP || actionMasked == MotionEvent.ACTION_CANCEL || (actionMasked == MotionEvent.ACTION_DOWN && !result)) { stopNestedScroll(); } return result; } ``` 看到dispatchTouchEvent，我们可能会想，这个方法名看着像是分发事件的方法，View不是仅仅消费事件吗，还需要处理分发？其实不是这样的，因为View对于事件可以有选择的，可以选择不处理事件，那么就会往上派给父类去处理这个事件，如果能够消费，那么就在onTouchEvent中处理了。 **核心代码1**：首先拿到一个ListenerInfo对象，这个对象中标记了这个View设置的监听事件，这里有几个判断条件：（1）ListenerInfo不为空，而且设置了OnTouchListener监听；\ （2）设置了OnTouchListener监听，而且onTouch方法返回了true 这个时候，result设置为true； **核心代码2**：如果满足了核心代码1的全部条件，那么核心代码2就不会走到onTouchEvent这个判断条件中，因为result = true不满足条件直接break。那么如果设置了OnTouchListener监听，而且onTouch方法返回了false，那么result = false，核心代码2就能够执行onTouchEvent方法，我们看下这个方法实现。 ```java public boolean onTouchEvent(MotionEvent event) { //...... if (clickable || (viewFlags & TOOLTIP) == TOOLTIP) { switch (action) { case MotionEvent.ACTION_UP: mPrivateFlags3 &= ~PFLAG3_FINGER_DOWN; if ((viewFlags & TOOLTIP) == TOOLTIP) { handleTooltipUp(); } if (!clickable) { removeTapCallback(); removeLongPressCallback(); mInContextButtonPress = false; mHasPerformedLongPress = false; mIgnoreNextUpEvent = false; break; } boolean prepressed = (mPrivateFlags & PFLAG_PREPRESSED) != 0; if ((mPrivateFlags & PFLAG_PRESSED) != 0 || prepressed) { // take focus if we don't have it already and we should in // touch mode. boolean focusTaken = false; if (isFocusable() && isFocusableInTouchMode() && !isFocused()) { focusTaken = requestFocus(); } if (prepressed) { // The button is being released before we actually // showed it as pressed. Make it show the pressed // state now (before scheduling the click) to ensure // the user sees it. setPressed(true, x, y); } if (!mHasPerformedLongPress && !mIgnoreNextUpEvent) { // This is a tap, so remove the longpress check removeLongPressCallback(); // Only perform take click actions if we were in the pressed state if (!focusTaken) { // Use a Runnable and post this rather than calling // performClick directly. This lets other visual state // of the view update before click actions start. if (mPerformClick == null) { mPerformClick = new PerformClick(); } if (!post(mPerformClick)) { performClickInternal(); } } } if (mUnsetPressedState == null) { mUnsetPressedState = new UnsetPressedState(); } if (prepressed) { postDelayed(mUnsetPressedState, ViewConfiguration.getPressedStateDuration()); } else if (!post(mUnsetPressedState)) { // If the post failed, unpress right now mUnsetPressedState.run(); } removeTapCallback(); } mIgnoreNextUpEvent = false; break; case MotionEvent.ACTION_DOWN: if (event.getSource() == InputDevice.SOURCE_TOUCHSCREEN) { mPrivateFlags3 |= PFLAG3_FINGER_DOWN; } mHasPerformedLongPress = false; if (!clickable) { checkForLongClick( ViewConfiguration.getLongPressTimeout(), x, y, TOUCH_GESTURE_CLASSIFIED__CLASSIFICATION__LONG_PRESS); break; } if (performButtonActionOnTouchDown(event)) { break; } // Walk up the hierarchy to determine if we're inside a scrolling container. boolean isInScrollingContainer = isInScrollingContainer(); // For views inside a scrolling container, delay the pressed feedback for // a short period in case this is a scroll. if (isInScrollingContainer) { mPrivateFlags |= PFLAG_PREPRESSED; if (mPendingCheckForTap == null) { mPendingCheckForTap = new CheckForTap(); } mPendingCheckForTap.x = event.getX(); mPendingCheckForTap.y = event.getY(); postDelayed(mPendingCheckForTap, ViewConfiguration.getTapTimeout()); } else { // Not inside a scrolling container, so show the feedback right away setPressed(true, x, y); checkForLongClick( ViewConfiguration.getLongPressTimeout(), x, y, TOUCH_GESTURE_CLASSIFIED__CLASSIFICATION__LONG_PRESS); } break; //----------注意这里的返回值，clickable为true----------// return true; } //----------注意这里的返回值，clickable为false----------// return false; } ``` 这里就是对所有事件的处理，包括但不限于ACTION_DOWN、ACTION_UP，**我们需要知道一点就是，View的click事件其实是在ACTION_UP中处理的**。我们从上面的源码中可以看出来，在ACTION_UP中有一个方法performClickInternal，具体实现为performClick方法。 ```java public boolean performClick() { // We still need to call this method to handle the cases where performClick() was called // externally, instead of through performClickInternal() notifyAutofillManagerOnClick(); final boolean result; final ListenerInfo li = mListenerInfo; if (li != null && li.mOnClickListener != null) { playSoundEffect(SoundEffectConstants.CLICK); li.mOnClickListener.onClick(this); result = true; } else { result = false; } sendAccessibilityEvent(AccessibilityEvent.TYPE_VIEW_CLICKED); notifyEnterOrExitForAutoFillIfNeeded(true); return result; } ``` 在这个方法中，我们貌似看到同样的一段代码，如果设置了OnClickListener监听，那么就会执行onClick方法也就是响应点击事件。所以通过上面的分析，我们能够了解，如果同一个View同时设置了setOnClickListener和setOnTouchListener，如果setOnTouchListener返回了false，那么点击事件是可以响应的；如果setOnTouchListener返回了true，那么点击事件将不再响应。 ```kotlin binding.tvHello.setOnClickListener { Log.e("TAG","OnClick") } binding.tvHello.setOnTouchListener(object : View.OnTouchListener{ override fun onTouch(v: View?, event: MotionEvent?): Boolean { Log.e("TAG","onTouch") return false } }) ``` 还需要注意一点的就是，对于clickable这个属性要求非常严格，必须要设置为true才可以进行事件的消费，也就是说**在clickable为true的时候，onTouchEvent才会返回true，否则就会返回false**，这个DOWN事件没有被消费，也就是说在dispatchTransformedTouchEvent方法中返回了false，此时就不会给 mFirstTouchTraget == null 赋值，后续MOVE事件进来就不会处理，这里需要非常注意。这里伙伴们如果不理解，可以换句话说：就是当DOWN事件来临之后，其实ViewGroup一定会将事件分发给子View，看子View要不要消费，如果子View不是clickable的，也就是说clickable = false，那么此时子View的onTouchEvent返回false，那么dispatchTouchEvent也是返回false，代表子View不消费这个事件，那么此时dispatchTransformedTouchEvent也是返回了false，mFirstTouchTraget还是空；因为**子View没有消费DOWN事件，那么后续事件不会再触发了**。 ## 1.3 ViewGroup的事件分发 -- ACTION_DOWN 前面我们介绍了View对于事件的消费，不管是click还是touch，都有对应的标准决定是否能够响应事件，最终View的dispatchTouchEvent返回值，就是result的值，只要有一个事件被消费，那么这个事件就算是到头了，但是，如果最终事件没有被消费，也就是说dispatchTouchEvent返回了false，那么父容器就能够拿到这个状态，决定谁去处理这个事件。所以ViewGroup就像是荷官，卡牌就是事件，她可以决定牌发到谁的手里，所以ViewGroup的事件分发机制核心就在于dispatchTouchEvent方法。 ```java public boolean dispatchTouchEvent(MotionEvent ev) { // ...... if (onFilterTouchEventForSecurity(ev)) { final int action = ev.getAction(); final int actionMasked = action & MotionEvent.ACTION_MASK; // Handle an initial down. if (actionMasked == MotionEvent.ACTION_DOWN) { // Throw away all previous state when starting a new touch gesture. // The framework may have dropped the up or cancel event for the previous gesture // due to an app switch, ANR, or some other state change. cancelAndClearTouchTargets(ev); resetTouchState(); } // Check for interception. final boolean intercepted; if (actionMasked == MotionEvent.ACTION_DOWN || mFirstTouchTarget != null) { final boolean disallowIntercept = (mGroupFlags & FLAG_DISALLOW_INTERCEPT) != 0; if (!disallowIntercept) { intercepted = onInterceptTouchEvent(ev); ev.setAction(action); // restore action in case it was changed } else { intercepted = false; } } else { // There are no touch targets and this action is not an initial down // so this view group continues to intercept touches. intercepted = true; } // If intercepted, start normal event dispatch. Also if there is already // a view that is handling the gesture, do normal event dispatch. if (intercepted || mFirstTouchTarget != null) { ev.setTargetAccessibilityFocus(false); } // Check for cancelation. final boolean canceled = resetCancelNextUpFlag(this) || actionMasked == MotionEvent.ACTION_CANCEL; // Update list of touch targets for pointer down, if needed. final boolean isMouseEvent = ev.getSource() == InputDevice.SOURCE_MOUSE; final boolean split = (mGroupFlags & FLAG_SPLIT_MOTION_EVENTS) != 0 && !isMouseEvent; TouchTarget newTouchTarget = null; boolean alreadyDispatchedToNewTouchTarget = false; // -------- 这里是不拦截的时候会走的地方 -------// if (!canceled && !intercepted) { // If the event is targeting accessibility focus we give it to the // view that has accessibility focus and if it does not handle it // we clear the flag and dispatch the event to all children as usual. // We are looking up the accessibility focused host to avoid keeping // state since these events are very rare. View childWithAccessibilityFocus = ev.isTargetAccessibilityFocus() ? findChildWithAccessibilityFocus() : null; if (actionMasked == MotionEvent.ACTION_DOWN || (split && actionMasked == MotionEvent.ACTION_POINTER_DOWN) || actionMasked == MotionEvent.ACTION_HOVER_MOVE) { final int actionIndex = ev.getActionIndex(); // always 0 for down final int idBitsToAssign = split ? 1 << ev.getPointerId(actionIndex) : TouchTarget.ALL_POINTER_IDS; // Clean up earlier touch targets for this pointer id in case they // have become out of sync. removePointersFromTouchTargets(idBitsToAssign); final int childrenCount = mChildrenCount; if (newTouchTarget == null && childrenCount != 0) { final float x = isMouseEvent ? ev.getXCursorPosition() : ev.getX(actionIndex); final float y = isMouseEvent ? ev.getYCursorPosition() : ev.getY(actionIndex); // Find a child that can receive the event. // Scan children from front to back. final ArrayList preorderedList = buildTouchDispatchChildList(); final boolean customOrder = preorderedList == null && isChildrenDrawingOrderEnabled(); final View[] children = mChildren; for (int i = childrenCount - 1; i >= 0; i--) { final int childIndex = getAndVerifyPreorderedIndex( childrenCount, i, customOrder); final View child = getAndVerifyPreorderedView( preorderedList, children, childIndex); // If there is a view that has accessibility focus we want it // to get the event first and if not handled we will perform a // normal dispatch. We may do a double iteration but this is // safer given the timeframe. if (childWithAccessibilityFocus != null) { if (childWithAccessibilityFocus != child) { continue; } childWithAccessibilityFocus = null; i = childrenCount - 1; } if (!child.canReceivePointerEvents() || !isTransformedTouchPointInView(x, y, child, null)) { ev.setTargetAccessibilityFocus(false); continue; } newTouchTarget = getTouchTarget(child); if (newTouchTarget != null) { // Child is already receiving touch within its bounds. // Give it the new pointer in addition to the ones it is handling. newTouchTarget.pointerIdBits |= idBitsToAssign; break; } resetCancelNextUpFlag(child); if (dispatchTransformedTouchEvent(ev, false, child, idBitsToAssign)) { // Child wants to receive touch within its bounds. mLastTouchDownTime = ev.getDownTime(); if (preorderedList != null) { // childIndex points into presorted list, find original index for (int j = 0; j < childrenCount; j++) { if (children[childIndex] == mChildren[j]) { mLastTouchDownIndex = j; break; } } } else { mLastTouchDownIndex = childIndex; } mLastTouchDownX = ev.getX(); mLastTouchDownY = ev.getY(); newTouchTarget = addTouchTarget(child, idBitsToAssign); alreadyDispatchedToNewTouchTarget = true; break; } // The accessibility focus didn't handle the event, so clear // the flag and do a normal dispatch to all children. ev.setTargetAccessibilityFocus(false); } if (preorderedList != null) preorderedList.clear(); } if (newTouchTarget == null && mFirstTouchTarget != null) { // Did not find a child to receive the event. // Assign the pointer to the least recently added target. newTouchTarget = mFirstTouchTarget; while (newTouchTarget.next != null) { newTouchTarget = newTouchTarget.next; } newTouchTarget.pointerIdBits |= idBitsToAssign; } } } //-------------这里是拦截之后会走的地方-------------// // Dispatch to touch targets. if (mFirstTouchTarget == null) { // No touch targets so treat this as an ordinary view. handled = dispatchTransformedTouchEvent(ev, canceled, null, TouchTarget.ALL_POINTER_IDS); } else { // Dispatch to touch targets, excluding the new touch target if we already // dispatched to it. Cancel touch targets if necessary. TouchTarget predecessor = null; TouchTarget target = mFirstTouchTarget; while (target != null) { final TouchTarget next = target.next; if (alreadyDispatchedToNewTouchTarget && target == newTouchTarget) { handled = true; } else { final boolean cancelChild = resetCancelNextUpFlag(target.child) || intercepted; if (dispatchTransformedTouchEvent(ev, cancelChild, target.child, target.pointerIdBits)) { handled = true; } if (cancelChild) { if (predecessor == null) { mFirstTouchTarget = next; } else { predecessor.next = next; } target.recycle(); target = next; continue; } } predecessor = target; target = next; } } // Update list of touch targets for pointer up or cancel, if needed. if (canceled || actionMasked == MotionEvent.ACTION_UP || actionMasked == MotionEvent.ACTION_HOVER_MOVE) { resetTouchState(); } else if (split && actionMasked == MotionEvent.ACTION_POINTER_UP) { final int actionIndex = ev.getActionIndex(); final int idBitsToRemove = 1 << ev.getPointerId(actionIndex); removePointersFromTouchTargets(idBitsToRemove); } } if (!handled && mInputEventConsistencyVerifier != null) { mInputEventConsistencyVerifier.onUnhandledEvent(ev, 1); } return handled; } ``` ### 1.1.1 万事皆始于ACTION_DOWN 看着dispatchTouchEvent这么长的代码，是不是脑袋都昏了，我给伙伴们分下层，首先一切的事件分发都是从ACTION_DOWN事件开始，所以我们可以看下ACTION_DOWN事件是如何处理的。 **核心代码1：** ```java final boolean intercepted; if (actionMasked == MotionEvent.ACTION_DOWN || mFirstTouchTarget != null) { final boolean disallowIntercept = (mGroupFlags & FLAG_DISALLOW_INTERCEPT) != 0; if (!disallowIntercept) { intercepted = onInterceptTouchEvent(ev); ev.setAction(action); // restore action in case it was changed } else { intercepted = false; } } else { // There are no touch targets and this action is not an initial down // so this view group continues to intercept touches. intercepted = true; } ``` 当ACTION_DWON事件来了之后，首先调用ViewGroup的dispatchTouchEvent方法，在上面这段代码中，就是判断ViewGroup是否要拦截这个事件，如果DOWN事件都被拦截了，就没有小弟的份了。所以如果当前是DOWN事件，或者mFirstTouchTarget不为空。**首先这里有一个变量mFirstTouchTarget，我们可以认为这个就是可能会消费事件的View**，因为首次肯定为空，但是当前为DOWN事件，所以这个条件是满足的，那么就会进入到代码块中。在代码块中，有一个disallowIntercept变量，**这个变量标志着子View是否需要消费这个事件，如果需要消费这个事件，子View可以调用requestDisallowInterceptTouchEvent这个方法，设置为true，那么父容器就不会拦截。** 所以如果子View需要消费这个事件，那么disallowIntercept = true，这个时候intercepted = false，意味着父容器不会拦截；如果子View不消费这个事件，那么disallowIntercept = false，然后会判断ViewGroup中的onInterceptTouchEvent方法，是否由父容器消费这个事件从而决定intercepted的值。 **所以看到这里，其实我们在解决事件冲突的时候就会有两种方式：一种就是重写父容器的onInterceptTouchEvent方法，由父容器决定是否拦截；另一种就是由子View调用requestDisallowInterceptTouchEvent方法，通知父容器是否能够拦截。** 那么假设，当前ViewGroup要拦截这个事件，也就是在onInterceptTouchEvent中返回了true ```kotlin override fun onInterceptTouchEvent(ev: MotionEvent?): Boolean { return true } ``` ### 1.1.2 ViewGroup拦截事件 **那么既然拦截了事件，那么当前ViewGroup就需要决定到底处不处理事件**，如果不处理就需要向上传递。因为ViewGroup拦截了事件，因此intercepted = true，在1.3开头的代码中，我标记了2个位置，一个是拦截会走的位置，一个是没有拦截会走的位置。 **核心代码2：** ```java if (mFirstTouchTarget == null) { // No touch targets so treat this as an ordinary view. handled = dispatchTransformedTouchEvent(ev, canceled, null, TouchTarget.ALL_POINTER_IDS); } else { // Dispatch to touch targets, excluding the new touch target if we already // dispatched to it. Cancel touch targets if necessary. TouchTarget predecessor = null; TouchTarget target = mFirstTouchTarget; while (target != null) { final TouchTarget next = target.next; if (alreadyDispatchedToNewTouchTarget && target == newTouchTarget) { handled = true; } else { final boolean cancelChild = resetCancelNextUpFlag(target.child) || intercepted; if (dispatchTransformedTouchEvent(ev, cancelChild, target.child, target.pointerIdBits)) { handled = true; } if (cancelChild) { if (predecessor == null) { mFirstTouchTarget = next; } else { predecessor.next = next; } target.recycle(); target = next; continue; } } predecessor = target; target = next; } } ``` 因为这个时候，mFirstTouchTarget还是为空的，所以会调用dispatchTransformedTouchEvent方法。 ```java private boolean dispatchTransformedTouchEvent(MotionEvent event, boolean cancel, View child, int desiredPointerIdBits) { final boolean handled; // ...... // Perform any necessary transformations and dispatch. if (child == null) { handled = super.dispatchTouchEvent(transformedEvent); } else { final float offsetX = mScrollX - child.mLeft; final float offsetY = mScrollY - child.mTop; transformedEvent.offsetLocation(offsetX, offsetY); if (! child.hasIdentityMatrix()) { transformedEvent.transform(child.getInverseMatrix()); } handled = child.dispatchTouchEvent(transformedEvent); } // Done. transformedEvent.recycle(); return handled; } ``` **这时候需要注意一点，这个方法第三个参数为null；** **所以当child为空的时候，就会调用父类的dispatchTouchEvent，也就是View的dispatchTouchEvent方法**，在1.2小节中我们是对这个方法做过分析的，也是会决定是否处理这个事件，最终返回是否处理的结果。所以这一次的结果（handled的值）最终决定了当前ViewGroup是否会处理这个事件，如果不处理，那么就扔到上级再判断。 ### 1.1.3 ViewGroup不拦截事件如果ViewGroup不拦截事件，那么intercepted = false，所以会走到分发事件的代码中。 **核心代码3：** ```java if (!canceled && !intercepted) { if (actionMasked == MotionEvent.ACTION_DOWN || (split && actionMasked == MotionEvent.ACTION_POINTER_DOWN) || actionMasked == MotionEvent.ACTION_HOVER_MOVE) { final int actionIndex = ev.getActionIndex(); // always 0 for down final int idBitsToAssign = split ? 1 << ev.getPointerId(actionIndex) : TouchTarget.ALL_POINTER_IDS; // Clean up earlier touch targets for this pointer id in case they // have become out of sync. removePointersFromTouchTargets(idBitsToAssign); final int childrenCount = mChildrenCount; if (newTouchTarget == null && childrenCount != 0) { final float x = isMouseEvent ? ev.getXCursorPosition() : ev.getX(actionIndex); final float y = isMouseEvent ? ev.getYCursorPosition() : ev.getY(actionIndex); // Find a child that can receive the event. // Scan children from front to back. final ArrayList preorderedList = buildTouchDispatchChildList(); final boolean customOrder = preorderedList == null && isChildrenDrawingOrderEnabled(); final View[] children = mChildren; //----------遍历集合，从后往前取------------// for (int i = childrenCount - 1; i >= 0; i--) { final int childIndex = getAndVerifyPreorderedIndex( childrenCount, i, customOrder); final View child = getAndVerifyPreorderedView( preorderedList, children, childIndex); // If there is a view that has accessibility focus we want it // to get the event first and if not handled we will perform a // normal dispatch. We may do a double iteration but this is // safer given the timeframe. if (childWithAccessibilityFocus != null) { if (childWithAccessibilityFocus != child) { continue; } childWithAccessibilityFocus = null; i = childrenCount - 1; } //-----判断View是否有消费的可能性---------// if (!child.canReceivePointerEvents() || !isTransformedTouchPointInView(x, y, child, null)) { ev.setTargetAccessibilityFocus(false); continue; } newTouchTarget = getTouchTarget(child); if (newTouchTarget != null) { // Child is already receiving touch within its bounds. // Give it the new pointer in addition to the ones it is handling. newTouchTarget.pointerIdBits |= idBitsToAssign; break; } resetCancelNextUpFlag(child); //-------- 这个方法需要注意，第三个参数不为空----------// if (dispatchTransformedTouchEvent(ev, false, child, idBitsToAssign)) { // Child wants to receive touch within its bounds. mLastTouchDownTime = ev.getDownTime(); if (preorderedList != null) { // childIndex points into presorted list, find original index for (int j = 0; j < childrenCount; j++) { if (children[childIndex] == mChildren[j]) { mLastTouchDownIndex = j; break; } } } else { mLastTouchDownIndex = childIndex; } mLastTouchDownX = ev.getX(); mLastTouchDownY = ev.getY(); newTouchTarget = addTouchTarget(child, idBitsToAssign); alreadyDispatchedToNewTouchTarget = true; break; } // The accessibility focus didn't handle the event, so clear // the flag and do a normal dispatch to all children. ev.setTargetAccessibilityFocus(false); } if (preorderedList != null) preorderedList.clear(); } if (newTouchTarget == null && mFirstTouchTarget != null) { // Did not find a child to receive the event. // Assign the pointer to the least recently added target. newTouchTarget = mFirstTouchTarget; while (newTouchTarget.next != null) { newTouchTarget = newTouchTarget.next; } newTouchTarget.pointerIdBits |= idBitsToAssign; } } } ``` **这里首先会判断事件是否为down事件，只有down事件才会分发，如果是move或者up事件便不会分发**。所以伙伴们需要牢记一点，如果在某个控件上产生了up事件，即便是设置了onClickListener，因为没有接收到down事件，所以也不会响应点击事件。然后调用buildTouchDispatchChildList方法，对当前ViewGroup全部的子View根据Z轴顺序排序， ```java ArrayList buildOrderedChildList() { final int childrenCount = mChildrenCount; if (childrenCount <= 1 || !hasChildWithZ()) return null; if (mPreSortedChildren == null) { mPreSortedChildren = new ArrayList<>(childrenCount); } else { // callers should clear, so clear shouldn't be necessary, but for safety... mPreSortedChildren.clear(); mPreSortedChildren.ensureCapacity(childrenCount); } final boolean customOrder = isChildrenDrawingOrderEnabled(); for (int i = 0; i < childrenCount; i++) { // add next child (in child order) to end of list final int childIndex = getAndVerifyPreorderedIndex(childrenCount, i, customOrder); final View nextChild = mChildren[childIndex]; final float currentZ = nextChild.getZ(); // insert ahead of any Views with greater Z int insertIndex = i; while (insertIndex > 0 && mPreSortedChildren.get(insertIndex - 1).getZ() > currentZ) { insertIndex--; } mPreSortedChildren.add(insertIndex, nextChild); } return mPreSortedChildren; } ``` **这里我们可以看到是按照Z轴值从高到低排序，Z值越大，说明其层级越深，最终拿到一个View的集合**。然后遍历取值的时候，是按照倒序取值的方式，因为Z值越小，说明其层级越浅，事件被消费的概率就越高；**取出一个View之后，首先需要判断它是否具备消费事件的可能性。** ```java if (!child.canReceivePointerEvents() || !isTransformedTouchPointInView(x, y, child, null)) { ev.setTargetAccessibilityFocus(false); continue; } ``` 第一个条件：View是可见的，或者 getAnimation() != null ```java protected boolean canReceivePointerEvents() { return (mViewFlags & VISIBILITY_MASK) == VISIBLE || getAnimation() != null; } ``` 第二个条件：当前View在点击（x,y）的范围之内，如果离着手指点击的位置很远，肯定不可能消费。 ```java protected boolean isTransformedTouchPointInView(float x, float y, View child, PointF outLocalPoint) { final float[] point = getTempLocationF(); point[0] = x; point[1] = y; transformPointToViewLocal(point, child); final boolean isInView = child.pointInView(point[0], point[1]); if (isInView && outLocalPoint != null) { outLocalPoint.set(point[0], point[1]); } return isInView; } ``` 所以经过层层筛选，也就只剩下一小部分可能会消费事件的View，那么怎么把他揪出来呢？**经过筛选的View最终调用了dispatchTransformedTouchEvent方法，在1.1.2中我们介绍了这个方法，就是用来判断是否消费事件的，这里传入的第三个参数不为空！** **回到前面dispatchTransformedTouchEvent方法中，当child不为空的时候，走到else代码块中，最终还是调用了child的dispatchTouchEvent方法。** 所以如果当前View消费了DOWN事件，那么返回值为true，也就是dispatchTransformedTouchEvent返回了true，那么会进入下面代码块中。 ```java if (dispatchTransformedTouchEvent(ev, false, child, idBitsToAssign)) { // Child wants to receive touch within its bounds. mLastTouchDownTime = ev.getDownTime(); if (preorderedList != null) { // childIndex points into presorted list, find original index for (int j = 0; j < childrenCount; j++) { if (children[childIndex] == mChildren[j]) { mLastTouchDownIndex = j; break; } } } else { mLastTouchDownIndex = childIndex; } mLastTouchDownX = ev.getX(); mLastTouchDownY = ev.getY(); //----- 这里就是给mFirstTouchTarget赋值--------// newTouchTarget = addTouchTarget(child, idBitsToAssign); alreadyDispatchedToNewTouchTarget = true; break; } ``` 因为当前child消费了事件，那么我们前面提到的mFirstTouchTarget就是由child封装一层得来的，也就是调用了addTouchTarget方法，也就是说**当一个child消费了一个DOWN事件之后，mFirstTouchTarget就不再为空了。** ```java private TouchTarget addTouchTarget(@NonNull View child, int pointerIdBits) { final TouchTarget target = TouchTarget.obtain(child, pointerIdBits); target.next = mFirstTouchTarget; mFirstTouchTarget = target; return target; } ``` 如果全部都不处理，那么mFirstTouchTarget还是为空，走到下面还是会执行ViewGroup拦截事件的逻辑，也就是1.1.2中的逻辑，所以说，**如果全部的子View都不处理，其实跟ViewGroup拦截事件的本质是一致的**。 ## 1.4 ViewGroup的事件分发 -- ACTION_MOVE 前面我们介绍了ViewGroup对于ACTION_DOWN事件的分发处理，因为DOWN事件只有一次，MOVE可以有无数次，所以在处理完DOWN事件之后，就会有MOVE事件涌进来。所以还是回到前面的判断条件中，我们对于MOVE事件的分发，需要基于DOWN事件的处理； ```java final boolean intercepted; if (actionMasked == MotionEvent.ACTION_DOWN || mFirstTouchTarget != null) { final boolean disallowIntercept = (mGroupFlags & FLAG_DISALLOW_INTERCEPT) != 0; if (!disallowIntercept) { intercepted = onInterceptTouchEvent(ev); ev.setAction(action); // restore action in case it was changed } else { intercepted = false; } } else { // There are no touch targets and this action is not an initial down // so this view group continues to intercept touches. intercepted = true; } ``` **如果ViewGroup拦截了事件：** 那么mFirstTouchTarget == null，会走到else中，此时 intercepted = true，那么就会走到ViewGroup拦截逻辑中，会调用dispatchTransformedTouchEvent，第三个参数child == null，那么如果ViewGroup不消费不处理，就会交给上级处理。 **如果ViewGroup不拦截事件：** 那么mFirstTouchTarget != null，此时还是会判断子View是否拦截该事件，如果拦截，那么intercepted = true，还是会走上面的拦截逻辑；如果不拦截，那么intercepted = false，会走到ViewGroup不拦截事件的逻辑中。 ```java if (newTouchTarget == null && mFirstTouchTarget != null) { // Did not find a child to receive the event. // Assign the pointer to the least recently added target. newTouchTarget = mFirstTouchTarget; while (newTouchTarget.next != null) { newTouchTarget = newTouchTarget.next; } newTouchTarget.pointerIdBits |= idBitsToAssign; } ``` 因为**只有DOWN事件的时候，才会遍历View树，如果是MOVE事件，不会进入循环，不再分发**，而是走上面的逻辑，此时newTouchTarget == null 而且 mFirstTouchTarget不为空，此时会给newTouchTarget重新赋值，然后继续往下走。 ```java if (mFirstTouchTarget == null) { // No touch targets so treat this as an ordinary view. handled = dispatchTransformedTouchEvent(ev, canceled, null, TouchTarget.ALL_POINTER_IDS); } else { // Dispatch to touch targets, excluding the new touch target if we already // dispatched to it. Cancel touch targets if necessary. TouchTarget predecessor = null; TouchTarget target = mFirstTouchTarget; while (target != null) { final TouchTarget next = target.next; if (alreadyDispatchedToNewTouchTarget && target == newTouchTarget) { handled = true; } else { final boolean cancelChild = resetCancelNextUpFlag(target.child) || intercepted; if (dispatchTransformedTouchEvent(ev, cancelChild, target.child, target.pointerIdBits)) { handled = true; } if (cancelChild) { if (predecessor == null) { mFirstTouchTarget = next; } else { predecessor.next = next; } target.recycle(); target = next; continue; } } predecessor = target; target = next; } } ``` 因为mFirstTouchTarget != null，因此会走到else代码块中，因为alreadyDispatchedToNewTouchTarget是在事件分发时才赋值为true，所以在while循环中（一次循环，单点触控），会走else代码块，其实还是会调用dispatchTransformedTouchEvent方法判断是否处理事件，**所以这就形成了一条责任链，当一个View消费了DOWN事件之后，后续的事件系统默认都会给他消费**，除非特殊情况。 # 2 Android事件冲突处理基于Android事件分发机制，DOWN事件只会执行一次，而且只是做分发工作，而MOVE事件会有无数次，所以对于事件冲突来说，只能在MOVE事件中进行处理。 ```java if (actionMasked == MotionEvent.ACTION_DOWN || mFirstTouchTarget != null) { final boolean disallowIntercept = (mGroupFlags & FLAG_DISALLOW_INTERCEPT) != 0; if (!disallowIntercept) { intercepted = onInterceptTouchEvent(ev); ev.setAction(action); // restore action in case it was changed } else { intercepted = false; } } else { // There are no touch targets and this action is not an initial down // so this view group continues to intercept touches. intercepted = true; } ``` 针对这种分发机制，前面也提到了两种处理方式，要么在父容器的onInterceptTouchEvent中判断是否拦截事件，要么控制disallowIntercept的值，所以就出现了2种拦截法。 ## 2.1 内部拦截法此方式指的是在子View中，通过控制disallowIntercept的值，来让父容器决定是否拦截事件。 ```kotlin class MotionEventLayout( val mContext: Context, val attrs: AttributeSet? = null, val defStyleAttr: Int = 0 ) : FrameLayout(mContext, attrs, defStyleAttr) { override fun onInterceptTouchEvent(ev: MotionEvent?): Boolean { return true } } ``` 如果在父容器的onInterceptTouchEvent方法中返回true，那么down一定会被拦截而不会分发给子View，所以子View不会响应任何事件。 ``` class MotionEventChildLayout( val mContext: Context, val attrs: AttributeSet? = null, val defStyleAttr: Int = 0 ) : FrameLayout(mContext, attrs, defStyleAttr) { private var startX = 0 private var startY = 0 override fun dispatchTouchEvent(ev: MotionEvent?): Boolean { when (ev?.action) { MotionEvent.ACTION_DOWN -> { //不能被拦截 parent.requestDisallowInterceptTouchEvent(true) } MotionEvent.ACTION_MOVE -> { var endX = ev.rawX var endY = ev.rawY //竖向滑动 if (abs(endX - startX) > abs(endY - startY)) { parent.requestDisallowInterceptTouchEvent(false) } startX = endX startY = endY } } return super.dispatchTouchEvent(ev) } } ``` 所以使用内部拦截法时，对于DOWN事件不能被拦截，需要将requestDisallowInterceptTouchEvent设置为true，这样父容器在分发事件时，就不会走自身的onInterceptTouchEvent方法（此时无论设置true或者false都是无效的），intercepted = false，此时事件就会被分发到子View。然后在滑动时，如果父容器支持左右滑动，子View支持上下滑动，那么就可以判断：如果横向滑动的距离大于竖直方向滑动的距离，任务在左右滑动，此时事件处理交给父容器处理；反之则交给子View处理。 **这是我们理解中的处理方式，看着好像没问题，但是实际运行时发现无效！！** 我们明明设置了requestDisallowInterceptTouchEvent为true，为什么没生效呢？ ```java if (actionMasked == MotionEvent.ACTION_DOWN) { // Throw away all previous state when starting a new touch gesture. // The framework may have dropped the up or cancel event for the previous gesture // due to an app switch, ANR, or some other state change. cancelAndClearTouchTargets(ev); resetTouchState(); } ``` 通过源码我们发现，当DOWN事件触发之后，会清除所有的标志位，包括disallowIntercept，所以在使用内部拦截法的时候，我们需要保证外部容器不能拦截DOWN事件，其实这个不会有问题的，大不了所有的子View都不处理，最终再扔给你处理。 ```kotlin override fun onInterceptTouchEvent(ev: MotionEvent?): Boolean { if (ev?.action == MotionEvent.ACTION_DOWN) { super.onInterceptTouchEvent(ev) return false } return true } ``` 所以在父容器的onInterceptTouchEvent方法中，不能对DOWN事件进行拦截，这里返回了false。因为父容器没有拦截down事件，所以事件被分发给了子View（可以上下滑动），紧接着MOVE事件来了，全部交给了子View处理，这时的mFirstTouchTarget还是子View的。**如果用户手势改成了左右滑动，那么这个过程两者是如何完成转换的呢？** 此时，mFirstTouchTarget != null，action == MOVE，disallowIntercept = false，因为是move事件，所有标志位不会被清除，此时会走到这里。 ```java if (!disallowIntercept) { intercepted = onInterceptTouchEvent(ev); ev.setAction(action); // restore action in case it was changed } ``` 此时，**父容器的onInterceptTouchEvent返回的是true，要拦截子View的事件了，此时intercepted = true**，因为mFirstTouchTarget != null，所以在拦截逻辑里，是会走到else代码块中的。 ```java while (target != null) { final TouchTarget next = target.next; if (alreadyDispatchedToNewTouchTarget && target == newTouchTarget) { handled = true; } else { final boolean cancelChild = resetCancelNextUpFlag(target.child) || intercepted; if (dispatchTransformedTouchEvent(ev, cancelChild, target.child, target.pointerIdBits)) { handled = true; } if (cancelChild) { if (predecessor == null) { mFirstTouchTarget = next; } else { predecessor.next = next; } target.recycle(); target = next; continue; } } predecessor = target; target = next; } ``` 因为这个时候 intercepted = true，所以cancelChild = true，所以在dispatchTransformedTouchEvent方法中，第二个参数为true。 ```java private boolean dispatchTransformedTouchEvent(MotionEvent event, boolean cancel, View child, int desiredPointerIdBits) { final boolean handled; // Canceling motions is a special case. We don't need to perform any transformations // or filtering. The important part is the action, not the contents. final int oldAction = event.getAction(); if (cancel || oldAction == MotionEvent.ACTION_CANCEL) { event.setAction(MotionEvent.ACTION_CANCEL); if (child == null) { handled = super.dispatchTouchEvent(event); } else { handled = child.dispatchTouchEvent(event); } event.setAction(oldAction); return handled; } } ``` **这时会触发一个ACTION_CANCEL事件，这个事件是子View事件被上层拦截的时候触发的**，其实当前这个MOVE事件做的一件事，就是**执行了子View的cancel事件，然后将mFirstTouchTarget置为了空**；因为MOVE事件很多，所以下个MOVE事件进来之后，又会走到判断是否拦截的逻辑中。此时父容器会冷酷地拦截这些MOVE事件，原本属于子View的MOVE事件，而且不会往下分发，走到拦截逻辑中，因为此时mFirstTouchTarget为空，所以直接由自身决定是否消费，肯定消费了，因为在左右滑动，也就是这样完成的事件消费处理权的切换。 ## 2.2 外部拦截法那么对于外部拦截法，则是需要动态修改onInterceptTouchEvent的返回值，如果用户左右滑动，那么就拦截，onInterceptTouchEvent返回true，此时intercepted = true，就不再走事件分发流程了。 ``` class MotionEventLayout( val mContext: Context, val attrs: AttributeSet? = null, val defStyleAttr: Int = 0 ) : FrameLayout(mContext, attrs, defStyleAttr) { private var startX = 0f private var startY = 0f override fun onInterceptTouchEvent(ev: MotionEvent?): Boolean { var intercepted = false when (ev?.action) { MotionEvent.ACTION_DOWN -> { } MotionEvent.ACTION_MOVE -> { val endX = ev.rawX val endY = ev.rawY //竖向滑动 intercepted = abs(endX - startX) > abs(endY - startY) startX = endX startY = endY } } return intercepted } } ``` 相较于内部拦截法，外部拦截就显得比较简单了，完全由父容器发牌决定。